How to close windows on the interface?
Ahah you are dragging the corners of the panels, actually increasing their number.
Quick dirty mode: see that long row of tabs up in the screen? Click one of those and you will have a cleaner view.
But you should learn to manage panels: so to remove panels, just click a corner again, one in between two panels that you want to unify, and move the mouse on the panel that you want to REMOVE. The other panel will take its place, and you will have one panel less.
Do this with all the panels that you don't want.
All those horizontal panels down there at the bottom of the GUI can be removed this way.
You have also activated the QUAD VIEW in the topmost panel. To deactivate it, go with the mouse cursor over that panel and press CTRL + ALT + Q.